In wireless communication systems, signals, such as radio signals, are used for conveying information over an air interface from a transmitter to a receiver. The signals propagate through air from the transmitter to the receiver over a radio channel, and are typically modulated and encoded e.g. in order to be more resistant against the influence of the radio channel. As part of the process of decoding a wireless transmission, the radio channel over which the transmission was conveyed is often analyzed and characterized. Examples of characterization includes e.g. forming a channel estimate, determining time and frequency offset of the transmission relative to an internal reference, and/or determining directional information. The better the quality of the characterization, the better is the performance of the decoding process.
Having a good estimate, or, more general, characterization of the radio channel at the transmitter is also beneficial as such. For example, characterization may allow the transmitter to adapt transmission parameters, such as precoder weights, scheduling decisions, and/or parameters related to modulation and channel coding.
In 4G and also most likely in future wireless systems, characterization of the radio channel is obtained using reference symbols. Reference symbols are inserted in the data stream, but, unlike the data symbols, the reference symbols are known to the receiver a priori. The assumption taken is that the reference signals experience a radio channel that is related to the radio channel experienced by the data symbols.
It is well known that increasing the amount of reference signals in the transmission will increase the quality of the channel characterization. However, increasing the amount of reference signals, e.g. increasing reference signal density, power or similar, comes with a cost in terms of overhead.